<!DOCTYPE html>
<html>
<head lang="en">
    <meta charset="UTF-8">
    <title></title>
    <style>
        em:nth-child(3){
            background-color: red;
        }
    </style>
</head>
<body>
    <div>
        <span>spanspan</span>
        <p>ppppppppppp</p>
        <em>ememmmmmmmmmmmmm</em>
    </div>
</body>
</html>